The field of the invention is x-ray imaging systems, and particularly, automatic exposure control and exposure optimization for x-ray systems.
Automatic exposure control (AEC) is used in x-ray imaging equipment to control the exposure per image. The goal is to maintain image quality while minimizing patient exposure. The AEC develops a signal proportional to the x-ray flux into the image receptor. This signal is used to regulate the total exposure for each image either by terminating the exposure or by adjusting the x-ray flux rate. In this usage, the x-ray technique (kVp, spectral filter, focal spot, etc) is prescribed by the operator. Often these are preset and selected based on patient size and anatomy to be imaged. Thus, the role of the AEC is to regulate the correct total exposure.
There are several methods of automatic exposure control currently in use. One of these uses an ionization chamber detector placed between the patient and the imaging detector. The ion chamber detector can be composed of several separate chambers, in which case the exposure-control signal can come from any single chamber or a combination of chambers. One disadvantage of this type of detector is that some of the radiation that would otherwise contribute to signal in the image receptor is lost because of attenuation in the ion chamber. Such chambers must also be carefully constructed so that any variation in absorption over their area is small enough to preclude artifacts in the detected image.
In another AEC method an ion chamber is placed behind the image receptor. In this position it does not intercept x-rays used for imaging, but the available radiation, and thus the signal in the ion chamber, is reduced because of attenuation in the image receptor and any associated packaging or shielding. The thickness of the ion chamber could be increased to increase its sensitivity, but this would make the imaging system more bulky. Some AEC systems employ a scintillating screen coupled to a light sensor in place of the ion chamber.
Another AEC method, which is used with image intensifier based systems, collects some of the light from the image gate at the output of the image intensifier and detects the brightness level with a photosensor. A disadvantage of this AEC method is that the light-pickup device is placed in the image path. This can lead to interference of the image by the pickup device in some imaging situations.
X-ray imaging systems that employ a large area solid-state x-ray detector, such as that described in U.S. Pat. No. 4,996,413 entitled xe2x80x9cApparatus And Method For Reading Data from An Image Detector,xe2x80x9d cannot use the AEC method employed in image intensifier systems. Unlike an image intensifier system, there is no minified light image from which light can be conveniently collected. Also, one of the design objectives when using large area solid-state detectors is to reduce the bulk of the detector package. This makes the use of an ion chamber placed in front of or behind the image detector less desirable.
U.S. Pat. No. 5,751,783 describes automatic exposure control for an x-ray system using a large area solid state x-ray detector including an array of photodiodes located behind the x-ray image detector to measure photons passing there through. The resulting currents from selective ones of these photodiodes are combined to provide a signal used to control the x-ray exposure.
The x-ray technique is selected prior to patient imaging based on apriori knowledge of patient size and anatomical view. These technique tables are often provided by the equipment manufacturer and are often not optimized, nor ideal for a particular patient or anatomy. Other times the operator does not select the appropriate anatomical view or patient size being imaged. The result can be an image with poor image quality or the wrong patient exposure. An automatic means for x-ray technique optimization and exposure control is needed.
The preferred embodiment is useful in an x-ray system for exposure control that generates an x-ray image of a patient while controlling the dose of x-rays received by a patient in order to generate the image. In such an environment, an apparatus embodiment comprises a source of x-rays and a digital detector arranged to generate detector data in response to the x-rays. An exposure control is arranged to generate data of interest within the detector data and to adjust the technique and/or dosage of x-rays to a predetermined level in response to the data of interest so that the image is generated to a predetermined image quality standard.
A comparable method also is included in another embodiment. By using the foregoing techniques, the image quality can be increased while the patient x-ray dose is controlled. In addition, system cost is reduced by eliminating the need for an ion chamber and system calibration time and operator errors also are reduced. The techniques provide an AEC signal without producing image artifacts or significantly increasing the total acquisition time or significantly increasing patient x-ray dose.